In a computer system, for improving the usage efficiency of the storage capacity, the technology by which the storage apparatus dynamically assigns actual storage areas (hereinafter referred to as pages) to logical devices provided to the server computer exists. Hereinafter, the logical device realized as described above is referred to as a virtual volume. There is also a device management method which configures a pool in which multiple media of different performances and bit costs are combined.
The operation of causing a pool configured of multiple logical resources to correspond to the virtual volume and, in accordance with the I/O (input/output) status from the host computer to the data written to the virtual volume, migrating the relevant data from the logical resource (pool volume) which assigns segments (pages) to the virtual volume to the segments of another pool volume is being performed (refer to the Patent Literature 1).